1. Field of the Invention
The present invention relates to a device for detecting ground faults in an inverter device for driving a brushless motor used in an electric power steering device for a vehicle and, more particularly, to a motor-driving-circuit ground-fault detection device for detecting ground faults in connection lines between a motor and an inverter circuit.
2. Description of the Related Art
An electric power steering device for a vehicle is provided with a motor such as a three-phase brushless motor, in order to apply, to a steering mechanism, a steering aiding force corresponding to the steering torque of a handle. The steering torque applied to the handle is detected by a torque sensor and, on the basis of the detected value, target values of electric currents to be flowed through the motor are calculated. Then, on the basis of the deflections between the target values and the values of the electric currents actually flowing through the motor, command values for feedback control to be supplied to a motor driving portion are calculated. The motor driving circuit includes an inverter circuit provided with upper and lower pairs of switching devices in association with the respective phases, and a PWM circuit for creating PWM (Pulse Width Modulation) signals having duty ratios corresponding to the command values and then switching control of the ON/OFF time intervals of the switching devices. The inverter circuit outputs voltages for the respective phases which correspond to the duty ratios, on the basis of the ON/OFF operations of the switching devices, to drive the motor with the voltages. The electric currents in the respective phases of the motor are detected by determining the voltages between the opposite ends of electric-current detection resistances connected in serial to the respective switching devices, and these detected values are the values of the electric currents actually flowing through the motor. Detections of phase currents in such a PWM type motor driving device are described in, for example, Japanese Patent Publication No. 3245727 and Japanese Patent Publication No. 3240535. Further, by using the electric-current detection resistances, it is possible to detect an excessive electric current flowing through the motor driving portion due to some abnormalities, in the event that the electric current values exceed a predetermined excessive-electric-current detection range.
Japanese Patent Publication No. 3245727 describes a motor driving device including a three-phase brushless motor, an inverter circuit for driving the motor, a PWM circuit which performs PWM control on the inverter circuit and electric-current detection resistances for detecting the phase currents flowing through the motor, wherein there is provided a sample-and-hold circuit which samples and holds the voltages across the electric-current detection resistances for the respective phases, using PWM signals from the PWM circuit as sampling signals, and outputs the sampled-and-held signals as phase-current detection signals.
Japanese Patent Publication No. 3240535 describes provision of a circuit which generates timing signals in addition to the structure of Japanese Patent Publication No. 3245727, for starting sampling of the voltages across the electric-current detection resistances later than the timing when the switching devices in the inverter circuit are turned on and ending the sampling of the voltages across the electric-current detection resistances earlier than the timing when the switching devices in the inverter circuit are turned off, on the basis of the timing signals from the circuit.
Japanese Patent Application Laid-Open No. 6-233450 describes a structure including lower switching devices connected to one another at their ground connection terminals, a first electric-current detection resistance for detecting an excessive electric current which is connected between the connection point among the lower switching devices and the ground, upper switching devices connected to one another at their power-supply connection terminals and a second electric-current detection resistance connected between the connection point among the upper switching devices and a power supply.
Further, the second electric-current detection resistance is provided in order to detect an excessive ground-fault electric current flowing through the resistance and to detect, therefrom, the occurrence of a ground fault, in the event of a ground fault in connection lines between an inverter circuit and a motor.